Color purity adjustment utilizing a coil attached to the faceplate

ABSTRACT

A method of adjusting the colour purity in colour picture display tubes the screen of which is provided with phosphor dots of luminescent material which are impinged upon by electron beams so that they luminesce in different colours. At least one of the electron beams is put into operation, while the picture to be displayed is without picture content and a coil or an annular magnet through which a direct current flows is placed in front of the screen of the tube. The purity adjusting members, that is to say, the purity magnets and/or purity coils and the securing means of the deflection unit are adjusted in such a manner that a ball of the colour corresponding to the said electron beams is placed in a fixed position within the coil. The coil may alternatively serve for degaussing metal parts of the picture tube.

United States Patent 1 Gerritsen JuneS, 1973 [75] Inventor: Jan Gerritsen, .Emmasingel, Eindhoven, Netherlands [73] Assignee: U.S. Philips Corporation, New

York, NY.

22 Filed: Mar. 24, 1971 211 Appl.No.: 127,614

[30] Foreign Application Priority Data 3,450,930 6/1969 Lien ..335/211 X 3,524,093 8/1970 Burdick et al.. ....335/212 X 3,573,525 4/1971 Fuse ....335/210 X 3,588,583 6/1971 Cierierski 3,590,302 6/1971 Bussey 3,550,038 12/1970 Shizu ..335/210 Primary Examiner-Carl D. Quarforth Assistant Examiner-P. A. Nelson AttameyFrank R. Trifari [5 7] ABSTRACT A method of adjusting the colour purity in colour picture display tubes the screen of which is provided with phosphor dots of luminescent material which are impinged upon by electron beams so that they luminesce in different colours. At least one of the electron beams is put into operation, while the picture to be displayed is without picture content and a coil or an annular magnet through which a direct current flows is placed in front of the screen of the tube. The purity adjusting members, that is to say, the purity magnets and/or purity coils and the securing means of the deflection unit are adjusted in such a manner that a ball of the colour corresponding to the said electron beams is placed in a fixed position within the coil. The coil may alternatively serve for degaussing metal parts of the picture tube.

13 Claims, 11 Drawing Figures PATENILL JUN 5 I973 SHEET 1 0F 3 mvENTR.

JAN GERRITSEN AGE NT PATENTLLJLIH 51973 SHEET 2 UF 3 INVENTOR.

JAN GERRITSEN AGENT PATENTEUJUH 51975 3,737, 716

sum 3 UP 3 Fig.9

INVENTOR.

JAN GERRITSEN BY} K ms COLOR PURITY ADJUSTMENT UTILIZING A COIL ATTACHED TO THE FACEPLATE The invention relates to a method of adjusting the colour purity with the aid of colour purity adjusting members in a colour picture display tube. The screen of, the tube is provided with phosphor dots of luminescent material which are impinged upon by at least two electron beams so that they luminesce in different colours. On the neck of the tube color purity magnets or coils and a deflection unit for deflecting the electron beams are provided. The colour purity adjusting members include the said magnets and coils or the securing means of the deflection unit. At .least one electron beam is put into operation during said adjustment, while the picture to be displayed is without any picture content.

The screen of the colour picture display tube currently most used, the shadow mask tube, includes a large number of phosphor dots which luminesce in the colours red, green and blue when they are impinged upon by electrons and which constitutes so-called triplets which are groups of three dots each of which luminesce in one of these three colours. An apertured mask is provided behind the screen and the number of apertures therein is three times smaller than that of the dots. The triplets, the apertures in the mask and the three electron guns are placed in such a manner that each phosphor dot is impinged upon by the relevant electron beam. However, to compensate for the effect of tolerance, use is made of so-called colour purity adjusting magnets which are provided on the neck of the tube. They are, for example, two annular rotatable permanent magnets with which both the direction and the intensity of a magnetic correction field may be adjusted. It is alternatively possible to use coils for this purpose, through which coils adjustable currents flow, or to use the combination of niagnets and coils. As a result it is ensured that each electron beam originates from the correct deflection point and therefore lands on the phosphor dots of the relevant colour. When the purity is correctly adjusted, a uniform colour is visible on the screen when displaying a picture without any picture content, a so-called white raster.

In the publication Philips Product Note No.5: Colour purity Adjustment" two methods have been described for adjusting the purity, namely the microscope method and the so-called redball method. In the firstmentioned method the location of the spot triplets are observed with the aid of a microscope and an auxiliary light source. These triplets are constituted by the landings on the screen of the electron beam and they are observed relative to the phosphor dot triplets in the middle of the screen in the case of a white raster with the aid of which the purity magnets are adjusted.

In the red-ball method the deflection unit which is the combination of the vertical and of the horizontal deflection coils is moved away as far as possible from the correct position so that a mislanding is produced beyond the centre of the screen. By causing only the "red electron gun to scan a raster, a red ball surrounded by faulty hues is produced on the screen. The adjusting procedure consists in adjusting the colour purity magnets in such a manner that this ball is moved to the centre of the screen. Subsequently the deflection unit is returned to the correct axial position and is secured so that a more or less correct landing on the entire screen is effected.

It is to be noted that in order to counter the detrimental effect of isotropic astigmatic deflection coils on the landing at the ends of the central lines of the screens, namely the disturbance of the equilateral shape of the triangles constituted by the phosphor dot triplets, the landings and the phosphor dots in the centre of the screen are not adjusted concentrically to each other in some types of tubes. To this end the construction of the picture tube is sometimes such that the landings are compressed" towards each other in the centre of the screen so that a satisfactory landing with as much tolerance as possible is obtained at the ends of the axes and especially at the edges of the screen. For this reason the red ball is not adjusted relative to the centre of the screen in this case, but rather slightly down to the left in the so-called 8 oclock direction. All this has been described in the said publication.

In practice the microscope method is not often used. Not only is a microscope with securing means necessary for this purpose, but it is usually impossible for one man to look through the microscope and simultaneously adjust the magnets on the rear side of the tube. In addition this method cannot easily be used for tubes in which the incoming electron beam is thicker than the phosphor dot, which dot is either or not surrounded by an absorbing material. Such a picture tube is described in U. S. Pat. No. 3,146,368.

The red-ball method is fairly generally used because it does not require a microscope and because one man may perform this method with the aid of a mirror. However, this method is not accurate. The centering of the fairly blurred ball in the centre of the screen without any further auxiliary means is not very exact and manufacturing tolerances of the picture tube may cause serious errors in the resultant adjustment because it is not effected at the correct axial position of the deflection unit. These errors may even render it impossible to correctly adjust some picture tubes, which of themselves are satisfactory.

The present invention relates to a novel adjusting method by which the said drawbacks are obviated, that is to say, which is more accurate than the red-ball method and which likewise as this method may also be used for tubes in which the electron beam is thicker than the phosphor dots. The method according to the invention may be performed by one man in a simple manner and is characterized in that a pole of at least one device for generating a magnetic field is placed in front of the screen of the display tube and that the purity adjusting members are adjusted in such a manner that balls of the colour corresponding to the electron beams put into operation are placed in previously determined positions within the field.

The device which may be used for performing the method according to the invention is very simple and cheap and is characterized in that it includes a source which provides a direct current to a coil and which may also include a thermistor having a positive temperature coefficient and a switch with which the coil can be arranged in series with the thermistor, while the series arrangement formed may be fed by an alternating voltage source.

In order that the invention may be readily carried into effect, some embodiments thereof will now be described in detail, by way of example with reference to the accompanying diagrammatic drawings, in which:

FIG. 1 is an elevational view of a picture tube including a device required for the method according to the invention,

FIG. 2 is a cross section of this picture tube seen from the side,

FIGS. 3 and 4 show the effect of the device of FIG.

FIG. 5 shows the mutual situation of a phosphor dot triplet and a landing triplet in the center of the screen of a picture tube employing pre-compression,

FIG. 6 shows an electron beam when the known redball method is used,

FIG. 7 is an elevational view of a picture tube in a further embodiment of the method according to the invention,

FIG. 8 is a cross section of this picture tube seen from above,

FIG. 9 shows a system of coils in a modification of the method according to the invention, while FIG. 10 shows the principle circuit diagram of a device with the aid of which the method according to the invention may be performed.

As is shown in FIG. 1, a substantially circular thin coil 2 is adjusted in front of the screen 1 of a picture tube, the axis of which coil coincides with that of the tube. Coil 2 is kept in position by means of a jig or is suspended for the upper rim of the cabinet of a colour television receiver incorporating the picture tube to be adjusted or this coil is secured in a different manner. Connections 3 of coil 2 are connected to a supply voltage apparatus not shown in FIG. 1, so that a direct current i determined by the copper resistance of the coil flows through coil 2, for example, in the direction which is shown in FIG. 1. The supply apparatus may of course alternatively be a current source. As a result the landing of the electron beam is not influenced in the centre of the screen, but elsewhere it is influenced. This may be clarified with reference to FIG. 2, showing the picture tube in a side-elevational view. Coil 2 generates a magnetic field some lines of force of which are shown in FIG. 2 in the direction which corresponds to the current direction in FIG. 1, which field is rotationsymmetrical relative to a terminal axis which coincides with the axis of coil 2. Coil 2 is so thin relative to its diameter that one side thereof generates a North pole and the other side generates a South pole, one of these poles engaging the substantially flat screen. The reference numeral 4 denotes one electron beam which is generated by an electron gun and is deflected by the deflection unit d which is substantially in its correct position and which electron beam subsequently passes through an aperture in the shadow mask. When the electron beam comes in the above-mentioned field it is deflected again thereby. The result is shown in FIG. 3. The electron paths undergo a rotation in the field generated by coil 2 so that mislandings occur and more of them occur as the distance to the centre of the coil is larger. FIG. 3 shows that the obtained displacement is effected in the same direction as that of current i. If this is reversed, the displacement of course also reverses its direction. The electron beams and the lines of force of the field are substantially parallel (FIG. 2) in the vicinity of the centre C of the screen so that the movement of the electrons is hardly influenced.

When operating only the red electron gun and when the picture to be displayed is a white raster a red ball is produced around the centre of coil 2, that is to say, the centre of the screen 1 if the purity magnets or purity coils p are correctly adjusted. This is shown in FIGS. 4a and 4b. FIG. 4a shows a red phosphor dot R, that is to say, a dot luminescing red light when it is impinged upon by electrons and which is located near the centre of the screen. Since the field generated by coil 2 does not substantially exert any influence at this area, the electron beam coincides substantially concentrically with the phosphor dot. FIG. 4b shows a triplet of three phosphor dots GB of red, green and blue luminescent materials, respectively, which are mutually arranged as is shown in the Figure as seen from the front side of the picture tube. The triplet in FIG. 4b is provided on screen 1 within coil 2 and in the vicinity thereof, at the upper left relative to the centre, for example, at point M of FIG. 3. As a result of the abovementioned rotation, the electron beam now lands in an area which is not concentrical with dot R, but as is shown in FIG. 4b. Thus this triplet luminesces in the colour magenta (purple). Other triplets may be considered in the same manner. For example, on the upper side the colour displayed is green instead of red and on the lower side it is blue.

The part of the screen where no mislanding occurs,

the red ball, is comparatively small so that the correct adjustment of the purity is much more accurate than in the known red-ball method. Coil 2 itself or a substantially circular line 5 (see FIGS. 1 and 3) provided within coil 2 may function as a centring template for this purpose. Alternatively a cross or the like may project the centre of coil 2, for example, on transparent paper. It may be noted that even if coil 2 or line 5 are nOt satisfactorily centered relative to the centre of the screen, this has a negligible influence on the adjusting accuracy, for an eccentricity of 1 cm of the coil causes a deviation of approximately 5 microns in a picture tube having a diagonal of 25 inches (63 cms) and a deflection angle of while a phosphor dot, as is known, has a diameter of approximately 400 microns. This may be explained by. the fact that the said deviation of 1 cm as seen from the deflection point corresponds to a very small angle.

It has been previously stated that in the known method the red ball is eccentrically adjusted when the picture tube has aconstruction such that the landing triplets and the phosphor dot triplets are not congruent in the centre of the-screen. A similar adjustment may alternatively be used in the method according to the invention. FIG. 5 shows a phosphor dot triplet which is located in the centre of the screen the desired location of the landing spots being indicated. This Figure shows that the red" landing spot must be centred around a point which is located down to the left at an angle of 30 relative to the horizontal line passing through the centre of the red phosphor dot. Since the purity must be adjusted in the centre of the screen, the landing must not be changed in that area due to the provision of coil 2. This means that coil 2 must in any case be centered around the centre C of the screen. The mentioned shift of the landing spot must be added to the rotation caused by coil 2 which rotation is shown in FIG. 3. FIG. 3 shows that the shift and the rotation compensate each other in a point N which relative to the centre of the screen is shifted down to the right at an angle of 60 relative to the horizontal, in the so-called 5 oclock direction. Therefore, the centre of the red ball must be moved towards this point. This applies when the current flowing through coil 2 flows in the direction shown in FIG. 3. If this current flows in the direction opposite thereto, the red ball must be shifted in the so-called 1 l oclock direction. It is possible to place line 5 which serves as a template or another mark eccentrically in order that the red ball can be placed accurately relative thereto.

The described adjusting method is very simple. One man may perform this method with the aid of a mirror, while the deflection unit need only be shifted in so far as this is necessary to compensate for spatial toleranees. lnaccuracies in the adjustment of the purity,

which might have been caused thereby, do not occur.

The greater accuracy of the method according to the invention as compared with the known red-ball method may be explained with reference to FIG. 6 in which the reference numeral 4 denotes an electron beam which lands on centre C or at a point in the vicinity thereof. Electron beam 4 is first deflected in the plane P containing the purity magnets P whereafter this beam passes through point A in the deflection plane D and is again deflected there. The beam then reaches point C on which a satisfactory landing is obtained provided that it has passed through the-correct deflection point A, that is to say, provided that it has been deflected in the plane P at the correct angle a. In the known redball method the purity magnets p are adjusted while deflection unit d is not in its correct position, that is to say, while the deflection plane is D (FIG. 5) in case of a retracted deflection unit. It is evident from FIG. 5 that the beam must pass through point A if the red ball is to be obtained in the centre of the screen and therefore it must also pass through point A in plane D. This results in the purity magnets being adjusted erroneously because the obtained deflection angle in the plane P is larger than a. If the red ball were produced by moving the deflection unit forwards, the purity magnets would have been adjusted to a too small correction in a corresponding manner. Only when the deflection unit is in the exact axial position, which is the case in the microscope method and in the method according to the invention it is possible to adjust the purity magnets without errors.

It is clear from the foregoing that the deflection unit d must be placed very accurately in the correct axial position (see FIG. 2) if the colour purity is to be satisfactory. If this is not the case, the results as regards the deflection itself are very small because only the sensitivity of the deflection coils is slightly changed thereby, which may be compensated by varying the amplitudes of the deflection currents. In fact, the deflection unit is then almost, that is to say, but for a few millimeters in its optimum position. To place deflection unit d in the position required for the colour purity after purity magnets are adjusted in the manner described, that is to say, after the purity is satisfactory in the centre of the screen, the following operations may be carried out. Unit d is axially moved until the reproduced colour on the entire screen is even and the exactness of the position found can be checked by putting a further electron beam or, which is more accurate, by putting the three electron beams into operation, for slight deviations are then visible as a discoloration.

The publication already quoted (see FIG. 6 of this publication) describes a more accurate method which, however, requires a microscope. According to one aspect of the invention a method to be preferred is therefore to move coil 2 horizontally, for example, to the left relative to the centre of screen 1. In FIG. 7 coil 2 occupies the position which is denoted by the reference numeral 2. A rotation of the landing spots occurs as was the case in FIG. 3 but with the difference that this rotation is not symmetrical relative to the centre of coil 2 but is symmetrical relative to a point 0' which relative thereto is located more closely to the axis of the picture tube. The electron beam impinging upon the screen at the area of the centre of coil 2' is in fact not parallel to the lines of force ofthe magnetic field generated by coil 2'. If the deflection unit is in its correct position, the landing on point 0 is satisfactory, in other words the red ball appears around this point Q which point is located on the horizontal center line. FIG. 8 shows a cross section of the picture tube seen from above, in which the solid line represents the electron beam impinging upon point Q when deflection unit d is in its correct position. If this unit is moved too far towards the screen, the electrons follow the path which is shown in a broken line and they land to the left of point Q. The same happens for all points which are present within coil 2. A movement to the left is thus superimposed on the above-mentioned rotation with the result that the red ball appears below point Q if current i for coil 2' flows in the indicated direction. On the other hand, when the deflection unit is too far remote from the screen, the red ball is noticeable above point Q. Consequently, if the red ball is centred around the point Q, the mechanical securing means s (FIGS. 2 and 8) of deflection unit d may be fixed. It may be noted that the adjustment of the purity magnets and the exact positioning of the deflection unit are then completely independent of each other.

The method described hereinbefore may be refined by using two similar coils 2' and 2" and by placing them against the screen 1 as is shown in FIG. 7. Two red balls are then observed which must be centred around the points Q and Q" located on the horizontal center line. When the deflection unit is moved, one red ball is moved upwards and the other is moved downwards if the current directions are those shown in FIG. 7, which facilitates the accurate adjustment. If one of the currents flow in a direction which relative to FIG. 7 is opposite, the two red balls are moved simultaneously upwards or downwards.

It may be noted that coils 2' and 2" need not necessarily be located around the horizontal axis of the screen, but may alternatively be placed, for example, around the vertical axis or around a diagonal.

As is apparent from the publication already mentioned, the landing spots and the phosphor dots must, however, not be concentric in the vicinity of points Q and Q". It may therefore occur that the deflection unit is exactly positioned for the red but not for the green colour purity. It is possible to adjust first for red, then for green and then again for red, which is not very practical. It may, however, be noted that the yellow ball which is produced when the red and the green electron guns are put into operation must be symmetrical relative to the vertical symmetry axis of the relevant phosphor dots. It is therefore more practical to make use of a yellow ball in one of the manners described with reference to the red ball.

The use of two coils 2 and 2" as in FIG. 7 has the drawback that a portion of the said coils protrudes beyond the screen 1 which in practice has its drawbacks or may even be impossible. FIG. 9 shows a system of coils which is better in this respect and which consists of coils 2, 2' and 2" and 2" which are provided on a piece of cardboard having the size of the screen while apertures are provided in the piece of cardboard through which the differently coloured balls can be observed. These apertures thus serve for the positioning of the coloured balls. Coil 2 serves for the adjustment of colour purity magnets p. Coils 2' and 2" for the accurate positioning of deflection unit d are substantially semicircular. However, since their electrical centre of gravity faces the centre of the screen, the accuracy of this positioning would be less satisfactory because the influence of the position of the deflection unit is greatest where the deflection is greatest. For this reason, coil 2" is provided through which a current flows whose direction as well as the direction of the current flowing through coil 2 amplifies the action of coils 2 and 2" so that the red or yellow ball is located as closely as possible near the edge of the screen.

FIG. shows the principle circuit diagram of an embodiment of a device by which the adjustment of the purity according to the invention may be performed. The device is equipped with four buttons K K K and K in which K is an on-off switch. When button K is depressed, a constant direct voltage is produced across Zener diode 6 which voltage is obtained by means of a rectifier circuit coupled to the power main. Simultaneously, this direct voltage is applied to the connections 3 of coil 2 for the adjustment of the colour purity magnets. For the adjustment of the axial position of the deflection unit this direct voltage is additionally applied through switch K, to the connections of coils 2', 2", and 2". In this embodiment in which a 110 colour picture tube is adjusted coil 2 has a diameter of 20 cms while the magnetic potential amounts to 60 ampere turns. Coils 2 and 2" likewise have a diameter of 20 cms and a magnetic potential of 135 ampere turns while the magnetic potential of coil 2" amounts to 40 ampere turns. After the colour purity is adjusted in the described manner, the shadow mask and possibly other metal parts of the picture tube must be degaussed because it has quite a considerable remanence as a result of the magnetic field generated by the coils. This may be performed in a simple manner by using the coils as degaussing coils. To this end button K is depressed so that the system of coils is switched to the power main voltage in series with a thermistor 7 having a positive temperature coefficient (PTC). After several seconds the device is switched off by means of button K It is evident that the degaussing circuit of a television receiver incorporating the picture tube to be adjusted may alternatively be used so that thermistor 7 and button K, are omitted.

In the described method of adjusting the colour purity magnets or coils use was made of the red" electron gun so that a red ball was produced. It is evident that a similar method may be used for the green or the blue electron gun. This also applies when adjusting picture tubes in which the landing triplets and the phosphor dot triplets are not congruent in the centre of the screen. When, for example, a green-ball method is used it follows from FIGS. 3 and 5 that the green balls must be moved in the so-called 10 oclock direction in that case if the congruent direction is the one shown in FIG. 3. Similar methods as the ones described are also usable if the phosphor dots are mutually arranged in a manner different from the manner shown in FIG. 6 or, likewise as in the known red-ball method, when the electron beams are thicker than the phosphor dots.

It is to be noted that the shape of coils 2, 2' and 2 need not be limited to a circle and a semicircle, respectively. Coil 2 must only have a shape which is symmetrical relative to the centre C of the screen. The same effect, that is to say, the red ball, may alternatively be obtained when an annular permanent magnet instead of coil 2 is used, which magnet, likewise as coil 2, need not be circular. In that case the device shown in FIG. 10 is omitted and degaussing cannot be effected otherwise than by means of the relevant circuit in the television receiver. In order that the field generated by the magnet has the same properties as the field shown with reference to FIG. 2, the magnet must be such that an annular side thereof is provided with similar poles. This similarly applies to the coils in FIG. 9 which may alternatively be replaced by permanent magnets.

What is claimed is:

l. A method of adjusting the color purity of a color display tube comprising activating at least one electron beam in in said tube without impressing any video signal thereon, whereby at least one distinct area is displayed on the tube screen, generating at least one pole of a magnetic field in front of said screen, and adjusting color purity members so that said area is in a predetermined position on said screen while keeping a deflection unit substantially in its proper position for deflecting said beam.

2. A method as claimed in claim 1 wherein said activating step comprises activating only one of said beams, and said generating step comprises generating a rotationally symmetrical magnetic field relative to a selected axis substantially coinciding with the axis of the display tube.

3. A method as claimed in claim 2 wherein said adjusting step comprises moving an alignment mark on said generating means in a direction perpendicular to a line connecting the center of an adjusted landing spot of the activated electron beam to a relevant phosphor dot on said screen.

4. A method as claimed in claim 2 wherein said generating step further comprises generating said magnetic field near an edge of said screen substantially symmetrical relative to a symmetrical axis or diagonal of said screen and placing an alignment mark on said generating means on said axis or diagonal.

5. A method as claimed in claim 1 wherein said generating step comprises generating two magnetic fields on a symmetry axis of said screen near the respective edges of said screen.

6. A device for adjusting the color purity of a display tube having a screen, and neck, and at least one electron beam activated without a video signal to cause an area to luminesce on said screen, said circuit comprising a deflection unit disposed on said neck in the proper position for deflecting the electron beam from said gun, color purity adjusting means comprising means coupled to said deflection unit for securing said unit and color purity adjusting magnets disposed on said neck, and means for generating a magnetic field having a pole placed in front of said screen; whereby said adjusting means can be adjusted to place said area in a predetermined position on said screen while leaving said deflection unit in the proper position for deflection of said beam.

7. A device as claimed in claim 6 wherein said magnets comprise permanent magnets.

8. A device as claimed in claim 6 wherein said magnets comprise electromagnets.

9. A device as claimed in claim 6 wherein said field generating means comprises alignment marks.

10. A device as claimed in claim 6 wherein said field generating means comprises a substantially planar coil adapted to receive a direct current therethrough.

to receive alternating current.

5mg UNiT D' STATES PATENT OFFICE CERTIFICATE OF CORRECTION intent 37377l6 Dated June 5,

Inventor(s) JAN- GERRITSEN a It is certified that error appears in the above-identified patent {and that said Letters Patent are hereby corrected as shown below:

IN THE TITLE PAGE below "Foreign Application Priority Data" insert Sept. 12 1970 Netherlands. .7013515 Signed and sealed this 8th day of October 1974.

(SEAL) V Attest:

MCCOY M; GIBSON JR. c. MARSHALL DANN Atts'ting' Officer i Commissioner of Patents 5732 2 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,737,716 Dated Juno 5 19 73 Inventofls) JAN GERRITSEN It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

IN THE TITLE PAGE below "Foreign Application Priority Data" insert Sept. 12 1970 Netherlands. .7013515 Signed and sealed this 8th day of October 1974.

(SEAL) Attest:

McCOY M. GIBSON JR. r C. MARSHALL DANN Arresting" Officer Commissioner of Patents 

1. A method of adjusting the color purity of a color display tube comprising activating at least one electron beam in in said tube without impressing any video signal thereon, whereby at least one distinct area is displayed on the tube screen, generating at least one pole of a magnetic field in front of said screen, and adjusting color purity members so that said area is in a predetermined position on said screen while keeping a deflection unit substantially in its proper position for deflecting said beam.
 2. A method as claimed in claim 1 wherein said activating step comprises activating only one of said beams, and said generating step comprises generating a rotationally symmetrical magnetic field relative to a selected axis substantially coinciding with the axis of the display tube.
 3. A method as claimed in claim 2 wherein said adjusting step comprises moving an alignment mark on said generating means in a direction perpendicular to a line connecting the center of an adjusted landing spot of the activated electron beam to a relevant phosphor dot on said screen.
 4. A method as claimed in claim 2 wherein said generating step further comprises generating said magnetic field near an edge of said screen substantially symmetrical relative to a symmetrical axis or diagonal of said screen and placing an alignment mark on said generating means on said axis or diagonal.
 5. A method as claimed in claim 1 wherein said generating step comprises generating two magnetic fields on a symmetry axis of said screen near the respective edges of said screen.
 6. A device for adjusting the color purity of a display tube having a screen, and neck, and at least one electron beam activated without a video signal to cause an area to luminesce on said screen, said circuit comprising a deflection unit disposed on said neck in the proper position for deflecting the electron beam from said gun, color purity adjusting means comprising means coupled to said deflection unit for securing said unit and color purity adjusting magnets disposed on said neck, and means for generating a magnetic field having a pole placed in front of said screen; whereby said adjusting means can be adjusted to place said area in a predetermined position on said screen while leaving said deflection unit in the proper position for deflection of said beam.
 7. A device as claimed in claim 6 wherein said magnets comprise permanent magnets.
 8. A device as claimed in claim 6 wherein said magnets comprise electromagnets.
 9. A device as claimed in claim 6 wherein said field generating means comprises alignment marks.
 10. A device as claimed in claim 6 wherein said field generating means comprises a substantially planar coil adapted to receive a direct current therethrough.
 11. A device as claimed in claim 6 wherein said field generating means comprises a substantially annular permanent magnet having a unipolar annular side.
 12. A device as claimed in claim 10 wherein said tube comprises a shadow mask tube and said coil comprises a removable coil means for degaussing said tube and any metal parts surrounding said tube.
 13. A device as claimed in claim 12 further comprising means for supplying direct current to said coil; and a series circuit comprising said coil, a switch, a positive temperature coefficient thermistor, and means adapted to receive alternating current. 